FIGURE 3 Capacitive touch sensors are an exciting new application for FIM technology. The sensors are created by placing a carbon-based capacitive film between the decorative printing layer and the circuit traces in the FIM process. Parts are molded so that the sensors will be very close
to the surface. Markets include automotive, home appliances (right), and more. Photos courtesy of Canatu.
and rigid silicon chips into the FIM process promises reduced
weight and thickness, greater freedom in design, and a more
streamlined production process. FIM enables designs that
replace mechanical buttons and dials with virtual ones, creating a single surface that is easier to keep clean and dust-free,
while removing mechanical parts that may break down over
time. The automotive industry is taking advantage of these
benefits by incorporating electronics into FIM in commercial
vehicles that are either on the road now or will be soon.
A variety of functions can be added to parts formed using
FIM. Options include antennas, sensors, LEDs, and integrated
circuits (see Figure 2). FIM can incorporate a variety of sensors
that may detect proximity to illuminate a display or activate gesture-based controls, detect ambient light to make controls visible
in dim or bright conditions, or add capacitive touch to control
virtual switches or sliders. Once the part is injection-molded, any
discrete electronic components are encapsulated and therefore
mechanically and chemically protected from the environment.
Creating printed antennas is a relatively straightforward
addition to FIM, as printed antennas are common in RFID
tags. Even though such antennas are designed for flat substrates, it's not a big stretch to incorporate them into FIM
parts. Silver-based inks are printed into circular patterns to
create an antenna. The design need only be modified to accommodate the bending that occurs during the forming step.
Recent developments include a trend toward using aqueous
rather than solvent-based conductive inks for a more environ-
mentally friendly process. These inks, however, may not be
sufficiently conductive or flexible for FIM applications.
Capacitive touch sensing is an especially compelling application for FIM. Sensors are located very close to the surface
of the part, making it more sensitive. Performance can even
be enhanced sufficiently that the features work while the
user is wearing gloves. Canatu makes innovative films for
capacitive touch, aimed primarily at the automotive market
(see Figure 3). The films combine two forms of carbon:
carbon nanotubes and fullerenes, 60-atom, soccer ball-shaped
molecules also known as buckyballs. The films have the high
electrical and thermal conductivity of CNTs and the chemical
reactivity of fullerenes, allowing them to bond to other surfaces. They're placed in between the decorative printing layer
and conductive printed traces in the FIM process.
One example of capacitive touch sensing is Canatu's FIMbased transparent touch sensor that controls both power
windows and seat heating in a single panel. Canatu developed
this product in partnership with automotive interior system
manufacturer Faurecia.
DESIGN AND PROCESS CONSIDERATIONS
While there are clear motivations for combining printed electronics with FIM, the process is not without complications.
The FIM process flow is slightly more complex when adding printed electronics since the printed circuitry is produced
as a separate layer from the decorative printing. The added
PROCESS FLOW FOR FILM INSERT MOLDING WITH PRINTED ELECTRONICS
FIGURE 4
1. Screen print
2. Screen print conductive
decorative design
traces and/or capacitive
onto flat film
sensors onto film
behind decorative layer
Courtesy of MacDermid Enthone.
30
SCREENPRINTING
3. Heat and form
film into desired
3D shape
4. UV cure inks
and coatings
(optional)
5. Attach LEDs, sensor
6. Insert film into
chips, and/or integrated
mold and inject
circuits with adhesives
thermoplastic resin
(optional)
on back side of part

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